We examined the tyrosinase-mediated oxidation of NPrCAP and its subsequent binding to sulfhydryl compounds (thiols) in NPrCAP-treated melanoma tissues and demonstrated that NPrCAP is oxidized by tyrosinase to form a highly reactive ortho-quinone, (N-propionyl-4-S- cysteaminylcatechol, NPrCAQ; Figure 8), which then binds covalently to biologically relevant thiols including proteins through the cysteine residues. Here, TYR is linked to melanoma.